Electric lamps



Feb. 20, 1962 D. P. COOPER, JR 3,022,438

ELECTRIC LAMPS Filed Sept. 10, 1959 ALUMINUM COATED WITH CALCIUMFLUORIDE 4, 3: HYDROGEN CHLORINE I VOLATILIZED CARBON j ARGON MIXTURE TTANTALUM Q CARBIDE FILAMENT CALCIUM FLUO RIDE COATING mza af- StatesUnite iice 3,622,438 ELECTRIC LAMPS Dexter Cooper, Jr., Lexington, Mass,assignor to Polaroid Corporation, Cambridge, Mass., at corporation ofDelaware Filed Sept. 10, 1959, Ser. No. 840,495 22 Claims. (Cl. 313-422)Thisinvention relates to new and improved electric incandescent lampsadapted to be operated at relatively high filament temperatures andpossessing relatively long, useful opera-ting life at such hightemperatures.

This application is a continuation-in-part of my copending applicationSerial No. 559,394, filed January 16, 1956, now abandoned.

Objects of the invention are to provide incandescent lamps of thecharacter described in which the filament consists essentially oftantalum carbide and in which the envelope of the lamp contains anatmosphere comprising, at operating temperatures, an inert gas,hydrogen, carbon, and a halogen, such as chlorine, fluorine or bromine.

Other objects of the invention will in part be obvious and will in partappear hereinafter.

The invention accordingly comprises the product possessing the features,properties, and the relation of elements which are exemplified in thefollowing detailed disclosure, and the scope of the application of whichwill be indicated in the claims.

For a fuller understanding of the nature and obects of the invention,reference should be had to the following detailed description taken inconnection with the accompanying drawing, which is a representation of asection through a typical automobile headlamp embodying the features ofthe invention.

Present-day electric incandescent lamps are generally constructed with atungsten filament mounted in an evacuated atmosphere within the bulb orin an atmosphere comprising essentially inert gases. These inert gasesmay, for example, comprise a mixture of gases of which the major portionis argon, krypton or xenon. Inert gases are employed to reduceevaporation of the filament during lamp operation. Nevertheless, acommon cause ofv lamp failure is evaporation of filament material tosuch an extent that the filament is gradually eaten away and fails.Since the rate of evaporation varies directly with the temperature, thiskind of failure is especially prevalent in bulbs designed to operate atvery high temperatures. Rapid filament evaporation has long been aserious problem in incandescent lamp manufacture, since the efficiencyof the bulb as a source of illumination measured in candle power perwatt, for example, generally increases as the operating temperature ofthe filament is raised. I

This invention accordingly contemplatesthe use of an atmosphere theelements of which interact with each other and with a tantalum carbidefilament in such a manner that the filament does not deteriorate over anextended period of operation at high filament temperatures. longoperating life and high efiiciency.

In the preferred embodiment of this invention, shown in one applicationin the drawing, a tantalum carbide filament is positioned within anatmosphere 12 comprising, at operating temperatures, hydrogen, chlorine,and volatilized carbon, together with an inert gas such as argon. Thecomponents of the atmosphere may be provided by introducinghexachlorobenzene, for example, before the bulb is sealed. The filament10 may be suspended between tantalum carbide leads 14 which are in turnattached to nickel subleads 16; the subleads may be connected to asource of electric power outside the The invention thus yields a lampthat has both.

envelope 18. In one preferred embodiment, the filament seven mils indiameter and about one-half inch in length may be mounted on 40-miltantalum carbide leads within a standard T-20 envelope of about 270 cc.capacity, into which about 2.75 mg. hexachlorobenzene and 20 cc.hydrogen, at atmospheric pressure and room temperature, have beenintroduced, along with sufiicient argon to bring the total pressurewithin the bulb at least to slightly less than one atmosphere. The bulband components may be cleaned and prepared in ways well known to theart. It is desirable to coat the inner surface of the envelope with amaterial that will protect the glass from attack. This may be done byevaporating calcium fluoride, for example, upon the inner surface of theenvelope before the lamp is assembled.

The filament may be of any suitable configuration, whether straight,coiled, crimped, or otherwise shaped. Although any convenient method offorming a tantalum carbide filament or leads may be used, it may beconvenient to convert a tantalum filament and leads to tantalum carbideafter the bulb has been assembled.

For example, the bulb may be constructed with all elements identicalwith those disclosed in the preferred embodiment of this invention,except that the filament and leads may comprise essentially puretantalum. The

filament may then be converted to tantalum carbide by baking the bulb atabout 300 C. for about 15 minutes, with sufficient current passingthrough the filament to yield a filament temperature of about 3100 C. Inthis way, the filament and lead ends associated with it will besubstantially converted to tantalum carbide. Tests show that it isunnecessary to carbide the remaining portions of the leads.

Many other materials may be used for leads. For example, rods made ofcarbon, tungsten or gold plated nickel have proved satisfactory.

The proper atmosphere may be provided by a number of materials orsources. For example, single compounds, such as ethylenediaminehydrochloride, methylamine hydrochloride and the like are satisfactory.The necessary elements may also be provided by introducing into theatmosphere a combination of materials, such as ethyl- I ene or anotherhydrocarbon and a gaseous hydrogen halide, e.g. hydrogen chloride; or acombination of hydrogen, chlorine and any convenient hydrocarbon, e.g.methane, ethane, ethylene and-the like; or a combination of hydrogen andany convenient halogenated hydrocarbon such as benzene hexachloride, thepolyhalogen derivatives of methane, ethane, etc., e.g. carbontetrachloride, tetrachloroethane and the like; or a combination ofhydrogen and polyhalogenated organic compounds such astetrachloroethylene and the like. It is obvious that the desiredatmosphere thus may be obtained in any number of suitable ways. Otherinert gases, such as xenon or krypton, may be used instead of argon.

Relatively high pressures within the bulb will lengthen lamp life; it isdesirable to maintain the pressure during operation at or near thehighest level that the envelope can safely withstand. If the pressuregenerated by the reacting gases is great enough, the need for an inertgas is reduced.

Other halogens may be used in place of chlorine without departing fromthe invention; members of the class of halogens having an atomic weightof less than are especially effective. If fluorine is used, however,precautions must be taken to avoid decomposition of the bulb envelopeand attack upon other lamp elements. If elemental halogens are used inpreparing the lamp atmosphere, precautions should be taken to avoidinhalation or contact with skin and eyes.

In general, any combination of materials may be used that will provide,at operating temperatures, an atmosphere of volatilized carbon,hydrogen, and a suitable halogen, preferably chlorine, in the areasurrounding the filament. The atmosphere should be substantially free ofwater or oxygen; specifically, the oxygen content should be less thanthe order of five parts per million. The amount of carbon in theatmosphere should be sufficient to prevent the tantalum carbide filamentfrom decomposing into free tantalum and carbon. Hydrogen and chlorinemay be used in varying proportions; it is critical only that enoughhydrogen be present to prevent chlorine from attacking the bulbcomponents, and that the total amount of hydrogen and chlorine besufficient to combine with carbon atoms escaping from the regionsurrounding the filament to reduce to a minimum deposit of uncombinedcarbon upon the inner wall of the bulb or upon other exposed surfaces.For example, the employment of benzene hexachloride produces anatmosphere comprising an atomic ratio of approximately one carbon atomto one chlorine atom to six hydrogen atoms. If the atmosphere isprovided by a mixture of materials, such as a hydrocarbon and gaseoushydrogen chloride, a desirable atomic ratio is one carbon to threechlorine to five hydrogen. It is to be understood that these ratios maybe varied widely, within the limits previously specified, withoutdeparting from the invention.

If this invention is to be used in an automobile headlamp, the reflectorportion 29 of the lamp may be coated with any convenient reflectingsurface, such as suitably protected aluminum.

Tests indicate that incandescent lamps constructed according to thefeatures of this invention may be operated successfully for extendedperiods at temperatures of the order of 3500 K. Illuminatingetficiencies may be obtained that are much higher than the efiiciency ofconventional bulbs, i.e., efiiciencies of the order of two and a halftimes present-day efliciencies.

Since certain changes may be made in the above products Withoutdeparting from the scope of the invention herein involved, it isintended that all matter contained in the above description or shown inthe accompanying drawing shall be interpreted as illustrative and not ina limiting sense.

What is claimed is:

1. In an incandescent lamp, a filament comprising essentially tantalumcarbide positioned within at atmosphere comprising, at operatingtemperatures, a halogen having an atomic weight of less than 100,hydrogen, and vaporized carbon.

2. A lamp according to claim 1, wherein the atmosphere comprises anexcess by volume of halogen over vaporized carbon.

3. A lamp according to claim 1, wherein the atmosphere also comprises aninert gas of low heat conductivity.

4. A lamp according to claim 1, wherein the atmosphere comprises anexcess by volume of halogen over vaporized carbon and an excess byvolume of hydrogen over halogen.

5. A lamp according to claim 1, wherein the halogen comprises bromine.

6. A lamp according to claim 1, wherein the halogen comprises chlorine.

7. A lamp according to claim 6, wherein the chlorine is present in anamount in excess by volume of the amount of vaporized carbon present inthe atmosphere, and wherein the atmosphere also comprises an excess byvolume of hydrogen over chlorine, and an inert gas of low heatconductivity.

8. In an incandescent lamp, a filament comprising essentially tantalumcarbide positioned within an atmosphere comprising, at operatingtemperatures, bromine, hydrogen, vaporized carbon, and an inert gas oflow heat conductivity.

9. In an incandescent lamp, a filament comprising essentially tantalumcarbide positioned Within an atmosphere comprising, at operatingtemperatures, a halogen having an atomic Weight of less than 10 0,hydrogen, vaporized carbon, and an inert gas selected from the classconsisting of argon, xenon, and krypton.

10. In an incandescent lamp, a filament comprising essentially tantalumcarbide positioned within an atmosphere comprising a source of hydrogen,chlorine, and vaporized carbon, said source providing an excess byvolume of chlorine over the amount of vaporized carbon in the atmosphereand also providing an excess by volume of hydrogen over the amount ofchlorine in the atmosphere.

11. A lamp according to claim 10, wherein the source comprisesmethylamine hydrochloride.

12. In an incandescent lamp, a filament comprising essentially tantalumcarbide positioned within an atmosphere comprising, at operatingtemperatures, fluorine, hydrogen and vaporized carbon. I

13. In an incandescent lamp, a filament comprising essentially tantalumcarbide positioned within an atmosphere comprising, at operatingtemperatures, a halogen having an atomic weight of less than 100,hydrogen, and vaporized carbon, said atmosphere comprising said halogen,hydrogen, and vaporized carbon in a ratio of approximately 1 carbon atomto 3 halogen atoms to 5 hydrogen atoms.

14. A lamp, according to claim 13,- wherein the halogen compriseschlorine.

15. In an incandescent lamp, a filament comprising tantalum carbidepositioned within an atmosphere comprising at least one source ofhydrogen, at least one source of a halogen having an atom weight of lessthan 100, and at least one source of carbon.

16. A lamp according to claim 15, wherein said atmosphere compriseshydrogen and a chlorinated hydrocarbon.

17. A lamp according to claim 16, wherein said chlorinated hydrocarboncomprises a chlorinated derivative of methane.

18. A lamp according to claim 17, wherein said chlorinated derivative ofmethane comprises carbon tetrachloride.

19. A lamp according to claim 15, wherein said atmosphere comprises ahydrocarbon and a gaseous hydrogen halide. E

20. A lamp according to claim 19 wherein said hydrogen halide compriseshydrogen chloride gas.

21. A lamp according to claim 15, wherein said atmosphere compriseshydrogen, a halogen and a hydrocarbon.

22. A lamp according to claim 15 wherein said atmosphere compriseshydrogen and a brominated hydrocarbon.

References Cited in the file of this patent UNITED STATES PATENTS

1. IN AN INCANDESCENT LAMP, A FILAMENT COMPRISING ESSENTIALLY TANTALUMCARBIDE POSITIONED WITHIN AT ATMOSPHERE COMPRISING, AT OPERATINGTEMPERATURE, A HALOGEN HAVING AN ATOMIC WEIGHT OF LESS THAN 100,HYDROGEN, AND VAPORIZED CARBON.